Group-III nitride (often referred to as III-nitride, or III-N) compounds, such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of the III-nitride compounds also make them excellent candidates for applications in high-temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3) substrates. However, a current problem with the manufacturing of GaN thin films is that there is no readily available suitable substrate material whose lattice constant and thermal expansion coefficient closely matching that of GaN. Among the possible substrates for GaN, silicon substrates were explored. Silicon substrates are attractive for GaN growth given their low cost, large diameters, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon wafers promises easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
A conventional optoelectronic (a light-emitting diode) device 2 is shown in FIG. 1, which includes a substrate, an AlN layer on the substrate, a superlattice layer on the AlN layer, and a light-emitting diode (LED) on the superlattice layer. The superlattice layer is formed of stacked AlN and GaN layers having equal thicknesses. The LED includes an n-type III-nitride layer, an active layer, and a p-type III-nitride layer. The AlN layer has the function of reducing the stress caused by the difference in coefficients of thermal expansion (CTE) between the substrate and the overlying III-nitride layers. However, the AlN layer has a high resistivity, and its existence prevents the formation of vertical LEDs whose contacts are on opposite sides of the substrate. The contacts to the p-type and n-type III-nitride layers thus have to be formed on the same side of the substrate. To form desirable vertical LEDs, this issue has to be addressed.